The final calculation of a subsidy's size will be done after the fact by the IRS.

"Your actual tax credit will be calculated based on your actual income that next April [when you file your federal tax return]," Levitt explained.

Bottom line: Anyone who might get a bigger subsidy than they're eligible for will have to pay back the difference to the IRS.

And they may owe a penalty, too, since they must attest when applying for subsidies that they are not filing false information.

"If you report your income incorrectly, it will catch up with you because there's a reconciliation of these tax subsidies on your tax return. Come tax time you could see an enormous bill," said Linda Blumberg, a senior fellow at the Urban Institute's Health Policy Center.

Of course, it's also possible to unintentionally understate your income when applying for credits.

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Changing jobs, losing one, getting a raise or simply increasing or decreasing work hours means your year-end income could be very different than what you initially reported.

That's why, Blumberg said, it will be important for consumers to alert the exchange within a month of their income changing so their subsidy can be adjusted accordingly for the rest of that year.

Efforts to educate the public about how the exchanges will work is getting under way, with less than 80 days to go before the exchanges open for enrollment on Oct. 1.